Wind energy systems have gained more and more importance as a source of energy in recent years. As areas suitable for their economical operation are relatively scarce in a number of countries, a growing number of wind energy systems are located close to populated areas. As the running of wind turbines in wind energy systems causes noise, this leads to various problems, such as resistance from neighborhoods and the like. A variety of factors contribute to the overall noise emission of such systems, such as tonal noise emissions and aerodynamic noise emissions.
Methods for reducing the noise emissions of wind energy systems have long been discussed. For example, it has been proposed to reduce aerodynamic noise caused by the blades through a speed/torque control of the system in order to keep turbine speed low during certain time intervals, e.g. during night time. Others have proposed to reduce speed/torque of the wind turbines in a wind park individually in order to gain maximum overall performance of the system while avoiding having a part of the wind turbines running at significantly higher speeds than the average speed of all turbines.
It is well established that to limit the noise generated by a wind turbine blade, the maximum tip speed with which it rotates must be limited. The limitation of the tip speed requires a reduction in the rated power to limit the torque generated in the drive train of the turbine. This reduces the annual energy yield of the turbine. However, for higher wind speeds it is not necessary to limit the maximum tip speed of the blade, as the background noise of the wind (e.g., the ambient or background sounds of the wind itself in the areas surrounding the wind turbine) will cover even a larger noise generation of the wind turbine. Therefore, it would be beneficial to provide a system in which the turbine can be re-accelerated and the original rated power achieved when environmental conditions allow, thereby providing a system that meets a specific maximum noise level with minimal loss in annual energy yield.